While optoelectronic and/or electrochemical devices have been dominated by solid-state junctions, usually made from crystalline or amorphous silicon and profiting from the experience and material availability resulting from the semiconductor industry, there is an increasing awareness of the possible advantages of devices based on mesoscopic inorganic or organic semiconductors commonly referred to as “bulk” junctions because of their interconnected three-dimensional structure. These are formed for example from nanocrystalline inorganic oxides, ionic liquids, and organic hole transporters or charge transporting polymer devices, which offer the prospect of very low cost of fabrication without expensive and energy-intensive high-temperature and high-vacuum processes, compatibility with flexible substrates, and a variety of presentations and appearances to facilitate market entry, both for domestic devices and in architectural or decorative applications.
For the fabrication of many optoelectronic and/or electrochemical devices the use of solid organic semiconductors is not favoured. The sensitized solar cell is an example of such: The hybrid version of this cell comprises a nanostructured oxide film coated with a monolayer of light absorbing sensitizer and infiltrated with an organic semiconductor. It is the latter step which is far from perfect with the current best approach being to spin-coat a concentrated solution of hole-transporter on top of the nanoporous film, allowing the favourable interaction between the hole-transporter and the dye-coated surface to act as a driving force to aid infiltration during solvent evaporation. There are many “voids” in the film of organic semiconductor obtained in this way, and as can be seen from cross-sectional imaging it is difficult to obtain material infiltration through more than a few microns thickness, which is significantly less than the light absorption depth of the composite, typically ten microns.
Objectives of the present invention are the improvement and/or optimisation of the manufacturing of optoelectronic and/or electrochemical devices, of such devices in general if they comprise organic semiconductors, and in particular of the interface between mesoscopic solid semiconductors and organic semiconductors in such devices, especially in sensitised solar cells.
There have been some studies on semi-conducting polymer solutions and electrolytes containing organic semiconductors, in order to improve the contact between semiconductors involving a mesoscopic surface. However, these solutions and electrolytes entail a number of disadvantages, because encapsulation and leakage of high vapour pressure organic solvents present in these electrolytes can hardly be prevented and thus constitute a major challenge for the commercialization of many electrochemical and/or optoelectronic devices and in particular sensitized solar cells. In electrolytes, charge transport is obtained by material diffusion. It is an objective to provide charge transport by electronic motion.
EP 1 160 888 A1 discloses a photoelectric conversion device in which a mixture of two charge transporting materials is used as an electrically conducting agent. At a ratio of 40:60, the mixture has a TG of −1.4° C. Mixtures of chemically different hole transporting materials are generally disadvantageous, due to the different energy levels of the components. Furthermore, the devices disclosed in EP 1 160 888 A1 have extremely low power conversion efficiencies (η), which are generally below 0.02%.
Haridas et al., in Synthetic Materials 121 (2001), 1573-1574, disclose polymeric hole conducting materials (HTM) having low melting point as hole conductor system in dye sensitised solar cells. The polymers have a molecular weight of >8000 and low penetration of the HTM into the nanoporous layer was reported. As the authors concede, their concept did not work.
It is thus a further objective of the present invention to capitalise upon the advantages of electrolytes and solutions in optoelectronic and/or electrochemical devices, while avoiding the disadvantages of the same. In particular, it is an objective to have all the typical advantages of organic semiconductors while optimising the contact at the interface of a mesoscopic semiconductor and an organic semiconductor.
It is a further objective to provide an organic semiconductor that is liquid at temperatures of device fabrication (“processing temperature”) and optionally solid at device operating temperatures.